The present invention relates to a liquid crystal cell, which can have a homeotropic structure with a compensated birefringence for said structure. It more particularly applies to the production of data display means, such as watches or electronic pocket calculators.
Various liquid crystal displays or cells are known, examples thereof being given in FIGS. 1A to 2B. FIGS. 1A and 1B diagrammatically show a liquid crystal cell of the "helical nematic" type, which has a nematic liquid crystal layer 2 between two glass plates 3, 4 carrying not shown electrodes, as well as two linear polarizers 5, 6 arranged on either side of the assembly constituted by the layer and the two plates. The layer is arranged in such a way that the molecules 7 forming it have, when the cell is inoperative (FIG. 1A), i.e. when no voltage is applied between the electrodes, an arrangement parallel to the plates and a helical structure such that the molecules facing the polarizer for receiving an incident light are perpendicular to the maximum absorption axis of said polarizer and that the molecules facing the other polarizer are parallel to said axis, the maximum absorption axes of the polarizers also being either perpendicular (to obtain a positive black on white contrast) or parallel (to obtain a negative white on black contrast). When the cell is excited, i.e. when a voltage of an appropriate level is applied between the electrodes, the liquid crystal has a homeotropic structure, in which the molecules 7 are all parallel to the same direction 8 perpendicular to the two planes and called "homeotropy direction" (FIG. 1B).
FIGS. 2A and 2B show diagrammatically a liquid crystal cell of the "electrically controlled birefringent type", having a nematic liquid crystal layer 9 between two glass plates 10, 11, which are provided with not shown electrodes, as well as two polarizers 12, 13 arranged on either side of the assembly, constituted by the layer and the plates. The two polarizers are preferably circular or quasi-circular and complementary to one another, i.e. able to give polarization directions opposite to two light waves propagating in the same direction perpendicular to the two polarizers, and respectively falling on each of these. When the cell is inoperative, the liquid crystal has a homeotropic structure so that all the molecules 14 which form it are then parallel to the same direction 15 perpendicular to the two plates 11, 12 and also called "homeotropy direction" (FIG. 2a). When the cell is excited, the molecules are all inclined in a same direction forming an angle .alpha. with the homeotropy direction.
The helical nematic or electrically controlled birefringence type cells suffer from a disadvantage. When they are in their homeotropic structure and are observed obliquely, their contrast deteriorates and this increases as the observation angle increases and said contrast can even be reversed. The object of the present invention is to obviate this disadvantage.